1. Field of the Invention
The present invention pertains to LADAR systems, and, more particularly, to a LADAR receiver for use in a multi-beam LADAR system.
2. Description of the Related Art
Many military and civilian applications rely on optical techniques such as laser detection and ranging (“LADAR”). At a very high level, LADAR works much like the more familiar radio wave detection and ranging (“RADAR”), in which radio waves are transmitted into the environment and reflected back, the reflections giving range and position information for the objects that generate them. LADAR does roughly the same thing, but using light rather than radio waves. Although there are some significant differences in performance, they are similar in at least this one basic respect.
Some LADAR systems employ a laser signal that is a pulsed, split-beam laser signal. That is, the LADAR transmitter transmits a laser signal in short bursts rather than continuously. The laser signal is often split into several spaced apart beamlets. Each pulse of the single beam is split, and so the laser signal transmitted is actually a series of grouped beamlets. The beamlets are then reflected to a LADAR receiver that picks up the reflected beamlets so that they can be detected, conditioned, and processed.
To actually detect the reflected laser signal, many LADAR transceivers use avalanche photodiodes (“APDs”) because they are fast, high gain photo-multiplication devices. Although highly sensitive in the avalanche mode of operation, APDs can be unpredictable with non-linear and inconsistent responsivity between devices. These problems are exacerbated when APDs are used in multi-channel systems because their gain variation can make channel equalization difficult. Still further, the use of multiple APDs also means that the performance of the system as a whole hinges on the weakest APD, which statistically lowers performance. APDs are also expensive and their support electroncs can be complex. Since each detector requires duplication of electronics in a multi-channel receiver, reducing the required number of APDs is important to reducing overall cost and packaging volume in a multi-beam LADAR system.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.